The proposed studies will be directed toward identifying and characterizing the genetic and biochemical properties of the various regulatory mechanisms that are operative in the interferon system. To this end, we have isolated virus-resistant mutants of cultured mouse embryo fibroblasts. We propose experiments to determine the biochemical nature of the block in virus replication, the genetic properties of the virus-resistant phenotype and the involvement of regulatory components of the interferon system in the expression of this phenotype. Furthermore we propose experiments to probe the molecular basis of the induction of interferon, the establishment of the anitviral state and the expression of the antiviral state. These include determining the viral function(s) required for interferon induction by temperature-sensitive mutants of Sindbis virus, the intracellular structure of vaccinia virus double-stranded RNA, the interferon response in enucleated cytoplasms produced by cytochalasin B treatment and the effects of extracts of interferon-treatment cells on the virion-bound transcriptase activity of vaccinia virus.